In ancient Rome, the lustrum was a ritual purification of the entire Roman population after the census every five years.

The English word lustrum (LUSS-trum) refers to that ancient practice, but has also come to mean more generally “a period of five years.” So I guess you can say it’s been a lustrum since Sept. 11, 2001. (Latin lustrum, by the way, means “purification,” and related to such bright, resplendent, and lucid words as luster and illustrate — literally, “to illuminate.”)